Liquid chromatography-mass spectrometry (LC-MS) has become an established, widely used analytical technique. Electrospray ionization (ESI) is by far the most popular LC-MS ionization method due to its high sensitivity, robustness and extended sample molecular weight range. ESI is sometimes supplemented and complemented with atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization (APPI) which in some cases show better performance with relatively small and less polar compounds. However, ESI, APCI and APPI still suffer from limitations in having poor or no ionization of non-polar compounds, are characterized by non-uniform compound-specific response and are plagued by ion suppression effects. In addition, all these atmospheric pressure ionization (API) methods are soft techniques that usually produce protonated molecular ions, hence requiring expensive high resolution accurate mass MS for analyte identification and characterization.
Consequently, electron ionization (EI) with its fragment-rich mass spectral pattern can significantly benefit LC-MS through the provision of automated library based identification with sample compound names and structures at the isomer level. In addition, EI seems ideally suitable for the LC-MS identification of unknown compounds that are not in the library, due to its provision of extensive fragment ions information. EI-LC-MS can also facilitate faster LC-MS analysis through the elimination of ion suppression effects that plague ESI and/or APCI and it is characterized by superior response uniformity for improved quantitation of unknown compounds.
Thus, combining EI with LC-MS is clearly highly valuable if the past problems of particle beam EI-LC-MS of limited sensitivity, poor linear dynamic range and limited range of compounds amenable for analysis can be solved.
Cappiello and co-workers developed a miniaturized version of Particle Beam EI-LC-MS [1] and later-on developed a new approach named Direct EI [2] that was demonstrated in a range of applications. This Direct EI interface was based on thermally assisted spray at the entrance of the EI ion source without any nebulizing gas. Recently they further improved their EI-LC-MS interface in their Liquid EI [3] in which the sample is thermally vaporized in its mass spectrometer transfer line that is connected with an electron ionization ion source. However, all the currently available EI-LC-MS are based on in-vacuum or reduced pressure thermally assisted spray formation from the LC output liquid flow. Consequently, they are limited to low, sub 1 μL/min LC liquid solution output flow rate and have no vaporized gas split line [2, 3]. Furthermore, the use of thermally assisted spray is known to exhibit liquid sample line clogging problems due to thermal condensation and/or decomposition of portion of the sample compounds and their matrices. In addition, service to the clogged liquid delivery capillary requires lengthy LC-MS system venting and pump down. As a result, these approaches suffer from limited concentration sensitivity, poor robustness and require tedious service.
Seemann et al. [4] and Amirav [5] developed and described another EI-LC-MS interface and system that is based on the transfer of vaporized sample compounds into a supersonic nozzle, expanding these vaporized sample compounds plus vaporized solvent and nebulizing helium gas from the supersonic nozzle into a differentially pumped vacuum chamber that is equipped typically with a 250 L/s turbo molecular pump, skimming the generated supersonic jet with a skimmer and generating a collision free supersonic molecular beam (SMB) of vibrationally cold sample molecules. These cold sample molecules in the SMB are collimated and pass axially inside a unique design of a fly-through electron ionization ion source that has zero internal electric field and the ionized sample compounds are further collimated and 90° deflected into the mass analyzer of a mass spectrometer for their mass spectrometery analysis. While EI-LC-MS with SMB is an effective EI-LC-MS method is it expensive, complex and difficult to be coupled with existing quadrupole MS such as of GC-MS with their available standard in vacuum EI ion sources. In addition, the electron ionization mass spectra of cold molecules in the SMB are characterized by enhanced molecular ions and thus are perceived as having lower matching factors with the EI mass spectra libraries.
Thus, there is still a clear need for a low cost EI-LC-MS system that can accept and analyze standard LC-MS flow rates, is reliable, robust, easy to use and service and that can be based on existing EI ion sources.
An important downside of EI-LC-MS that impedes its development despite its clear and well known benefits is that while APCI and/or APPI can be easily exchanged with ESI either automatically or via a minor and fast change of the LC-MS hardware without system venting, EI-LC-MS due to its in-vacuum ion source requires a separate mass spectrometer system, which is expensive.
Currently, analytical mass spectrometry is divided into mostly GC-MS or LC-MS and only in rare cases can one find a mass spectrometer system of LC-MS with APCI that can provide both GC-MS and LC-MS in a single MS system. Even in these cases, changing their mode of operation requires a change of hardware such as physical movement of the heavy and bulky GC towards the MS of the LC-MS plus some additional hardware changes in the APCI chamber. On the other hand, there is no GC-MS with EI and LC-MS with EI in a single MS system. Clearly having such universal chromatography MS system of both LC and GC coupled with a single MS is highly desirable and beneficial since it can save cost, valuable laboratory bench space and reduce system maintenance and manpower owing to the need to operate one system as opposed to two systems. Preferably, such a system should also enable the mode of operation to be changed under software requiring only a click (or few clicks) of the mouse without any hardware change.